April 7, 2011: With the west tunnel excavation complete, workers operating the tunnel boring machine began mining the east tunnel.

Patrick Cashin / MTA

September 2009: Second Avenue subway project

Patrick Cashin / Metropolitan Transportation Auth

October 2009: Second Avenue Subway project

By Katie Cannon, NBC News:

A project 90 years in the making had better be impressive, and the Metropolitan Transport Authority's $4.5 billion Second Avenue Subway project does not disappoint.

The undertaking, which began in 2007, can be found nine stories underneath the streets of Manhattan. It’s the first line to be built in New York City since 1932, and the initial phase will run from 96th street to 63rd street. There are plans to eventually run the line all the way to lower Manhattan.

MTA photographer Patrick Cashin began making pictures of the construction as soon as there was a hole large enough to venture into.

“I think when you’re down there for all of 10 seconds, you know that this is a dangerous place to be,” Cashin said in an interview published on Flickr's blog.

Despite the danger, he visits the site every few months to document the progress of this engineering feat.

Sept. 22, 2011: Workers completed tunneling for the first phase of the Second Avenue Subway when the project's tunnel boring machine reached the Lexington Av-63 St station, breaking into the existing subway system. The 485-ton, 450-foot-long TBM used a 22-foot diameter cutter head to mine 7,789 linear feet in two tunnels, averaging approximately 60 linear feet a day.

Patrick Cashin / MTA

January 21, 2012: Second Avenue subway project

Patrick Cashin / MTA

March 20, 2012: Nine stories underneath the streets of Manhattan, workers build the cavern that will house the 72nd street station.

Patrick Cashin / MTA

March 7, 2012: 72nd street station cavern

Patrick Cashin / MTA

March 7, 2012: Workers use tags to check in and out of the work site.

Patrick Cashin / MTA

March 20, 2012: Crews work in the cavern that will house the 72nd street station.

Patrick Cashin / MTA

February 23, 2013: 86th street station

Patrick Cashin / MTA

March 19, 2013: The cavern that will house the 96th street station. A construction worker became trapped in the mud at this location and was rescued by fellow workers and the FDNY.

Patrick Cashin / MTA

April 2013: 86th street station

Patrick Cashin / MTA

April 2013: Looking up out of the construction site that will house the future 86th street station.